This invention relates to a method of seismic data processing for correlating P and S-wave seismic traces.
For many years, seismic exploration for oil and gas has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected grid point in a terrain or is an energy source capable of delivering a series of impacts or mechanical vibrations to the earth's surface. The acoustic waves generated in the earth by these sources are transmitted back from strata boundaries and reach the surface of the earth at varying intervals of time, depending on the distance and the characteristics of the subsurface traversed. These returning acoustic waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical signals. The plurality of geophones are arrayed in a selected manner to detect most effectively the returning acoustic waves and generate electrical signals representative thereof from which data may be deduced concerning the geological subsurface of the earth.
Recently, two particular types of waves, described below, have been utilized to obtain more data on the subsurface structure.
Elastic waves in a solid propagate according to several modes. One mode of propagation of elastic waves is a compressional wave, or "P-wave", in which particle motion within the solid is in a direction perpendicular to the wavefront. In another mode, the shear or "S-wave" mode, the particle motion within the solid is parallel to the wavefront. The S-wave can be one of two components, SH or SV, where the particle motion of the SH-wave is perpendicular to the SV-wave. Compressional and shear waves travel at different velocities in a given solid and the ratio of these velocities, V.sub.p /V.sub.s, is a function of and hence indicative of the material in which the waves are propagating.
Typically, V.sub.p /V.sub.s ratios are calculated as follows. First P and S-waves are generated at the surface, for example, by the technique in which a mechanical vibration is imparted to the earth by a sinusoidal plate motion or an inclined impulse hammer. Each wave is detected conventionally by geophones which generate electrical signals in the form of wiggle traces. These signals representing the P and S-wave seismic traces or sections are then processed in a manner which is more fully described below to correlate the two traces. That is, one trace is stretched or contracted to correspond to the time scale of the other trace. As noted above, the P-waves can travel much faster than the S-waves in materials. Thus, the P and S wave seismic traces which are recorded may be superimposed to pick out corresponding reflection events for a common subsurface reflection point. From the correlated P and S-traces, a common sedimentary interval (strata layer) is identified on each seismic trace. The respective P-wave and S-wave travel times .DELTA.T.sub.p, and .DELTA.T.sub.s in the interval are then measured between the peaks or troughs of reflection events delineating the interval. Under the assumption that the P-wave and S-wave velocities are constant within the interval, the V.sub.p /V.sub.s ratio for the interval is then EQU V.sub.p /V.sub.s =(1/.DELTA.T.sub.p)/(1/.DELTA.T.sub.s)=.DELTA.T.sub.s /.DELTA.T.sub.p.
Thus, the above interval measurement of V.sub.p /V.sub.s ratio is based on matching a single point, say the peak on each P-wave reflection event with a corresponding point on each S-wave reflection event; that is, each pair of points is intended to represent the same physical location in the subsurface. Error is introduced into the V.sub.p /V.sub.s calculation when a pair of these chosen points do not, in fact, match the same subsurface point. Therefore, error in the calculation arises from an inaccurate correlation of each trace, giving rise to difficulties in identifying corresponding reflection events on P and S wave traces. A difference in V.sub.p /V.sub.s with respect to different types of rocks is not very great, and accordingly, very accurate interval measurements of the V.sub.p /V.sub.s ratio are necessary before reliable determinations of the sub-surface geology can be made. In particular, it is found that V.sub.p /V.sub.s, which can vary between about 1.20 and about 2.50, must be measured with an accuracy above about 0.1 or better if the results are to be useful.
Prior P and S-wave correlation methods have not been highly accurate over the entire P and S wave seismic traces, causing inaccurate V.sub.p /V.sub.s ratio estimates to be obtained over most of the P and S wave seismic traces.
For example, the oldest and least accurate method of correlating P and S-waves involves assuming a constant V.sub.p /V.sub.s ratio of about 2 and ratioing the plotting scales for the P and S data traces accordingly. This method totally disregards the nonlinear nature of the V.sub.p /V.sub.s ratio, and thus gives a correlation making the matching of corresponding reflection events on each trace extremely difficult, if not impossible.
In another correlation method, as described in U.S. Pat. No. 4,422,165 of Thomas et al, various constant stretch factors associated with different ratios of compressional to shear wave velocities are applied to a P-wave reflection segment until a best correlation with the corresponding S-wave reflection segment is obtained. This correlation is then associated with a given constant V.sub.p /V.sub.s ratio for that segment. The optimization and accuracy of the correlation process, hence the precision of the V.sub.p /V.sub.s ratio, is then estimated by a maximum likelihood technique. This method is an improvement over the above method, but still has inaccuracies due to the application of constant ratios. Furthermore, the absence of a P or S-wave reflection event in a geological interval, containing information on P and S wave velocity contrasts in that interval, could not be correlated by the method described in Thomas et al.